Electro-optic voltage sensor for sensing voltage in an E-field

ABSTRACT

A miniature electro-optic voltage sensor system capable of accurate operation at high voltages. The system employs a transmitter, a sensor disposed adjacent to but out of direct electrical contact with a conductor on which the voltage is to be measured, a detector, and a signal processor. The transmitter produces a beam of electromagnetic radiation which is routed into the sensor where the beam undergoes the Pockels electro-optic effect. The electro-optic effect causes phase shifting in the beam, which is in turn converted to a pair of independent beams, from which the voltage of a system based on its E-field is determined when the two beams are normalized by the signal processor. The sensor converts the beam by splitting the beam in accordance with the axes of the beam&#39;s polarization state (an ellipse whose ellipticity varies between -1 and +1 in proportion to voltage) into at least two AM signals. These AM signals are fed into a signal processor and processed to determine the voltage between a ground conductor and the conductor on which voltage is being measured.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States has rights in this invention pursuant to contractnumber DE-AC07-94ID13223 between the U.S. Department of Energy andLockheed Idaho Technologies Company.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains generally to the field of voltage sensorsand more particularly to a voltage sensor system which utilizes thePockels electro-optic effect to measure voltage.

2. Background Art

High-accuracy measurement of high voltage has traditionally beenaccomplished using iron-core ferro-magnetic potential transformers.These devices have substantially limited dynamic range, bandwidth,linearity, and electrical isolation. During electrical fault conditionsthese transformers can conduct dangerous levels of fault energy todownstream instrumentation and personnel, posing an additionalliability.

A variety of optic sensors for measuring voltage have been developed inattempts to offer the power industry an alternative to the conventionaltransformer technology. Generally, these voltage sensor systems requirethat direct electrical contact be made with the energized conductor.This contact is made necessary by the use of a voltage divider which isutilized to connect the sensing element with the energized conductor onwhich a measurement is to be made. Direct electrical contact with theconductor may alter or interrupt the operation of the power system bypresenting a burden or load.

In addition to the disadvantages associated with direct electricalcontact with the energized conductor, prior art voltage sensor systemsare typically bulky, particularly in extremely high voltageapplications. This is true because the size of the voltage dividerrequired is proportional to the voltage being measured. The size of suchsystems can make them difficult and expensive to install and house insubstations.

Many prior art sensors are based upon the electrostrictive principlewhich utilize interferometric modulation principles. Unfortunately,interferometric modulation is extremely temperature sensitive. Thistemperature sensitivity requires controlled conditions in order toobtain accurate voltage measurements. The requirement of controlledconditions limits the usefulness of such systems and makes them unsuitedfor outdoor or uncontrolled applications. In addition, interferometricmodulation requires a highly coherent source of electromagneticradiation, which is relatively expensive.

Open-air E-field based sensors have also been developed, but lackaccuracy when used for measuring voltage because the open-air E-fieldused varies with many noisy parameters including ambient dielectricconstant, adjacent conductor voltages, moving conductive structures suchas passing vehicles, and other electromagnetic noise contributions.

Systems which utilize mechanical modulation of the optical reflectionwithin an optic fiber have also been developed. Among other drawbacks,the reliance of such systems on moving parts is a significant deterrentto widespread use.

It would therefore be an advantage in the art to provide a system whichdoes not require direct electrical contact with the energized conductor,is compact, operates in a variety of temperatures and conditions, isreliable, and is cost effective.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an aelectro-optic voltage sensor system which does not require contact witha conductor.

It is a further object of the present invention to provide such anelectro-optic voltage sensor system which is capable of use in a varietyof environ mental conditions.

It is a still further object of the present invention to provide such anelectro-optic voltage sensor system which can be employed to accuratelymeasure high voltages without use of dedicated voltage divisionhardware.

It is an additional object of the present invention to provide such anelectro-optic voltage sensor system which minimizes the electronicsneeded for implementation.

It is a further object of the present invention to provide a sensorsystem capable of being integrated with existing types of high voltagepower transmission and distribution equipment so as to reduce oreliminate the need for largest and-alone voltage measurement apparatus.

It is yet another object of the present invention to provide a sensorsystem capable of being integrated with existing types of powertransmission and distribution equipment.

These and other objects of the present invention will become more fullyapparent from the following description and appended claims or may belearned by the practice of the invention as set forth herein.

While the present invention is described in terms of a sensor system, itis to be understood that the subject apparatus and method may be used inany field of electrical or optical application. Those having ordinaryskill in the field of this invention will appreciate the advantages ofthe invention, and its application to a wide variety of electrical uses.

The above objects and others not specifically recited are realized in aspecific illustrative embodiment of an Electro-Optical Voltage SensorSystem whereby one may measure the voltage difference (or electricalpotential difference) between objects or positions. Voltage is afunction of the electric field (hereinafter "electric field" shall beindicated "E-field") and the geometries, compositions and distances ofthe conductive and insulating matter. Where, as in the presentinvention, the effects of an E-field can be observed, a voltagemeasurement can be calculated.

The invention employs a transmitter, a sensor, a detector, and a signalprocessor. The transmitter produces a beam of electromagnetic radiationwhich is routed into the sensor. Although this electromagnetic radiationused in the present invention can comprise any wavelengths beyond thevisible spectrum, the term "light" will be used hereinafter to denoteelectromagnetic radiation for the purpose of brevity. The beam undergoespolarization before it undergoes an electro-optic effect in thetransducing material of the sensor. In the polarized beam the light hasat least two components which propagate along at least two orthogonalaxes, thus forming at least two orthogonal planes within the beam. Theelectro-optic effect occurs when the sensor is placed into an E-field,and is observable as a phase differential shift of the orthogonal beamcomponents. The planes of propagation are the object of the differentialphase shift. The differential phase shift causes a corresponding changein the beam's polarization. The polarization change is in turn convertedinto a set of amplitude modulated (AM) signals of opposing polarity thatare transmitted out of the sensor. The detector converts the set ofoptical AM signals into electrical signals from which the voltage isdetermined by the signal processor.

The sensor processes the beam by splitting the beam in accordance withthe components of the orthogonal polarization planes into at least twoAM signals. In the preferred embodiment, these AM signals are thenconverted into digital signals, fed into a digital signal processor andmathematically processed into a signal proportional to the voltage whichproduced the E-field.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and ad vantages of the inventionwill become apparent from a consideration of the subsequent detaileddescription presented in connection with the accompanying drawings inwhich:

FIG. 1 is a diagram of the electro-optical voltage sensor system shownin a generalized high voltage application scenario. The diagram was madein accordance with the principles of the present invention.

FIG. 2 is a diagram of an alternative embodiment of the electro-opticalvoltage sensor system (in the same generalized application scenario)made in accordance with the principles of the present invention.

FIG. 3 is a diagram of the light polarizing beam splitter of FIGS. 1 and2.

FIG. 4 shows a partially sectioned side view of a collimator of FIG. 2and also FIGS. 7 and 8, below.

FIG. 5 is a diagram of one embodiment of the transducer of FIGS. 1 and2, wherein transducing material in only one orientation is shown.

FIG. 6 is a diagram of one embodiment of the transducer in FIGS. 1 and2, wherein transducing material in two orientations is shown.

FIG. 7a is a top-down schematic view of one embodiment of the sensorhead of FIG. 2.

FIG. 7b is a top-down schematic view of one embodiment of the sensorhead of FIG. 2.

FIG. 7c is a top-down schematic view of one embodiment of the sensorhead of FIG. 2.

FIG. 8 is a top-down schematic view of one embodiment of the sensor headof FIG. 1.

FIG. 9 is a cross-sectional, schematic view of one embodiment of theE-field originator of FIGS. 1 and 2, as a shielded joint.

FIG. 10 is a top perspective, partially schematic view of one embodimentof the E-field originator of FIGS. 1 and 2, as a terminator.

FIG. 11 is a cross-sectional side view of one embodiment of the E-fieldoriginator of FIGS. 1 and 2, as a bushing.

FIG. 12 is a sectional, partially schematic, side view of one embodimentof the E-field originator of FIGS. 1 and 2, as a shielded bus orshielded cable.

FIG. 13 is a sectional, partially schematic, side view of one embodimentof the E-field originator of FIGS. 1 and 2, as a gas or oil-insulatedswitchgear.

FIG. 14 is a sectional, partially schematic, side view of one embodimentof the E-field originator of FIGS. 1 and 2, as a duct-enclosed bus.

FIG. 15 is a side view of one embodiment of the E-field originator ofFIGS. 1 and 2, as a clamp-on apparatus.

FIG. 16 is an alternative embodiment of the electro-optical voltagesensor of FIG. 1 having a current detector.

DETAILED DESCRIPTION

A preferred embodiment in accordance with the present invention isillustrated in FIG. 1, which is a diagram of the electro-optical voltagesensor system. There is shown a transmitter 1 that transmits a beam ofelectromagnetic radiation (the beam is not shown in FIG. 1 but generallydesignated as 12 elsewhere) and a sensor head 4. In the preferredembodiment the beam is routed from the transmitter 1 to the sensor head4 by a polarization maintaining (PM) fiber 18. In an alternativeembodiment the PM fiber 18 is replaced by low-birefringence fiber.Another alternative embodiment whose cost/performance characteristic maybe quite different and better suited to certain applications, entailsreplacement of the PM fiber 18 by single-mode or multi-mode optic fiber.The PM fiber 18 directs the beam from the transmitter 1 into the sensorhead 4. Optical signals are routed from the sensor head 4 by a pair ofeither single-mode or multi-mode optical fibers, shown as 37 and 38. Theoptical fibers 18, 37, and 38 electrically isolate the sensor head 4from the transmitter 1 and the rest of the detection system, providingfurther protection to personnel and equipment from the dangers of highvoltage systems.

The sensor head of the preferred embodiment of FIG. 1 is shown ingreater detail in the top-down schematic view of FIG. 8, where thepreferred embodiment of the sensor head is generally depicted at 4. Thesensor head 4 comprises a polarizer 6, a translucent medium 13, atransducer 5, a retro-reflector generally depicted at 10, a polarizingbeam splitter 80 and beam reflector 14. A collimator 26 is connected tothe PM fiber 18 and transmits the beam 12 from the PM fiber 18 into thepolarizer 6. The polarizer 6 polarizes the beam 12 when the beam 12 istransmitted from the collimator 26; thus, the polarizer 6 polarizes thebeam 12 if the beam 12 is not polarized when it is transmitted from thecollimator 26, or re-polarizes the beam 12 if the beam 12 is polarizedwhen it is transmitted from the collimator 26. As a practical matter,the polarizer 6 could be eliminated or placed anywhere between thetransducer 5 and the transmitter of the beam 12, including anywherealong the PM fiber 18. In an alternative embodiment the PM fiber 18 canbe aligned with sufficient precision with respect to the optical axes ofthe transducing medium, to perform the function of (and eliminate theneed for) the polarizer 6. The beam 12 passes from the polarizer 6through the translucent medium 13 and into the transducer 5. Thetranslucent medium 13 comprises a non-conductive, non-birefringentmaterial, such as fused quartz or a similar substance, which provides apathway for the beam from the polarizer 6, which is preferably situatedoutside the E-field, to the transducer 5, which must reside within theE-field. The source of this E-field will be discussed shortly.

When the transducer 5 (also called the transducing medium) is in anon-zero E-field (not shown) it induces a "differential phase shift" toorthogonal beam components of the beam 12 through the Pockelselectro-optic effect, which will now be explained. In the polarized beam12 the light has at least two components which propagate along at leasttwo orthogonal planes, respectively, thus forming at least twoorthogonal planes within the beam 12. The phase of the components ineach plane of propagation are the object of a shift, relative to thephase of the component in the other plane, in the transducer 5. ThePockels electro-optic effect, which takes place in transducer 5, changesthe relative phases of the beam components by altering their respectivevelocities, and is observed in Pockels transducing crystals and similarmedia. The magnitude of the phase shift, called the "differential phaseshift", is proportional to the magnitude of the E-field. Thus, thePockels electro-optic effect is observed as a "phase differential shift"of the orthogonal beam components which is proportional to the magnitudeof the E-field. Due to the fixed coaxial structure of the devices inwhich the sensor head is to be installed, the magnitude of the E-fieldis proportional to the voltage. Therefore, the differential phase shiftis proportional to (and can be used to measure) the voltage of theE-field between energized conductor and ground conductor.

The phase shift between orthogonal components further manifests itselfas an alteration of the beam's polarization. Therefore, the beam may beconsidered either to be a differential phase shifted signal or anoptical polarization modulation signal. The polarization modulationsignal is used in the present invention because it can be detected usinglow-cost, components that are less susceptible to temperature,mechanical perturbations, and optical incoherence than those component sthat would be required to sense the differential phase shift directly.

In the practice of the present invention, the sensor head 4 or thetransducer 5 may be encased in a dielectric buffering material,generally depicted at 87 in FIGS. 7a, 7b,and 7c, to smooth thetransition geometry between the permittivity of the transducer 5 and thepermittivity of the surrounding media, which in most cases will be aninsulator. The dielectric buffering material promotes uniformity in theE-field, particularly around the edges of the transducer 5. Thisenhances uniform phase shift in the beam 12 passing through thetransducer 5, and minimizes voltage stress on the materials in andsurrounding the sensor head 4 as well, thereby increasing the probablemaximum operating lifetime of the entire system.

After the beam 12 has passed sequentially through the polarizer 6 (FIG.8), translucent medium 13 and transducer 5, it enters into theretro-reflector 10. The retro-reflector 10 comprises a reflectormaterial 9 which induces the beam to reflect and causes a quarter-waveretarding of the beam 12. The quarter-wave retarding property of thereflector material 9 induces a 1/4 wavelength shift in the orthogonalplanes of the beam 12. The 1/4 wavelength shift can be achieved byreflection of the beam 12 alone, if the material 9 is non-birefringent;alternatively, the material 9 can comprise birefringent reflectormaterial wherein the properties of the material help achieve the shift.

If a reflector material 9 having birefringence is used, the phaseshifting occurs when orthogonal components of the electromagnetic wavesof the beam are shifted (here by 1/4 a wave length) with respect to oneanother. This birefringence, which is inherent in some materials, is notdependent upon the E-field. In the preferred embodiment, a reflectormaterial 9 is used which exhibits a reflection-induced retardation todifferentially shift the relative phases of the beam components inorthogonal planes in the beam 12 by π/2 radians. The polarization of thebeam 12 incident upon the retro-reflector 10 depends upon the E-fieldpresent when the beam 12 makes a first pass through the transducer 5. Ifthere is an E-field present then there will be some differential phaseshift already present in the beam 12.

The π/2 retardation within the retro-reflector 10 biases the sensor'soverall resultant polarization such that zero E-field (and hence zerovoltage) corresponds to circular-polarized light, as no differentialphase shift is induced upon the beam 12 by the transducer 5. However,due to the location of the retro-reflector 10 in the sensor head 4, ifthe transducer 5 is in a non-zero E-field and induces a differentialphase shift in the beam 12, then the retro-reflector 10 will not convertlight from linear to circular-polarization, rather it will induceelliptical-polarization upon the beam 12, whose ellipticity will vary inproportion to the voltage. While laser light is used in the preferredembodiment, other forms of electromagnetic radiation could also be usedin the practice of the invention.

The reflection of the beam 12 in the retro-reflector 10 is in accordancewith the principle of the angle of incidence being the same as the angleof reflection. In the practice of the preferred embodiment of thepresent invention, the retro-reflector 10 is configured to cause a 180°change in the direction of the beam 12, thereby sending the beam backinto the transducer 5. Upon each reflection, a beam 12 may be induced toundergo a 1/8 wave length phase shift to produce a total of 1/4 wavephase shift, as in the preferred embodiment. One skilled in the artcould further induce a 1/4 wave length shift in beam 12 by combiningreflection and birefringence.

When the beam 12 reenters the transducer 5, it undergoes further phaseshift from the Pockels electro-optic effect. As shown in FIG. 8, thebeam 12 then passes through the translucent medium 13 and enters intothe polarizing beam splitter 80.

In the polarizing beam splitter 80, (also called an analyzer in theart), the beam 12 is separated in accordance with the respective axes ofits polarization ellipse into AM signals 35 and 36. The saidpolarization ellipse will exhibit an ellipticity ranging between -1 and+1, in proportion to voltage at any given time. Those skilled in the artwill note that an elliptic polarization whose ellipticity ranges between-1 and +1 can be described as ranging from a linear polarization alongone axis to a linear polarization along a second axis perpendicular tothe first axis, wherein the point at which ellipticity equals 0corresponds to circular polarization. As shown in FIG. 3, the major andminor axes of the polarization ellipse of beam 12 can be represented bytwo orthogonal components, indicated generally at 83. The beam splitter80 then separates the beam 12 into two components 84 and 85 whichcomprise the intensities along each of the two axes of the polarizationellipse shown as orthogonal components 83. The intensity of beamcomponents 84 and 85 will modulate conversely to one another in responseto modulations in the ellipticity of the beam's polarization. The beamcomponents 84 and 85 are two AM signals shown as 35 and 36,respectively, which are routed as shown in FIG. 1 into photo-detectors43 and 44, respectively.

The AM signals 35 and 36 are then passed by collimators 27 and 28, shownin FIG. 8, and routed through multi-mode or single-mode optic fibers 37,38. A beam reflector 14 may be used to aid in routing the AM signals 35and 36.

As shown in FIG. 1, first and second translucent fibers 37 and 38, areused to route the AM signals 35 and 36 from the sensing means into thedetector 41. First the AM signals 35 and 36 are routed into thephotodetectors 43 and 44 through the first and second translucent fibers37 and 38, respectively. In the preferred embodiment the first andsecond translucent fibers 37 and 38 comprise at least one optic fiber,wherein the optic fiber is selected from the group consisting of: asingle-mode optic fiber, and a multi-mode optic fiber. In thephotodetectors 43 and 44 the AM signals 35 and 36 become electricalsignals 45 and 46. The electrical signals 45 and 46 are routed into asignal processor 62, the final component of the detector 41, wherein adesired E-field characteristic is determined, particularly that ofvoltage.

To determine the voltage in the practice of the preferred embodiment ofthe present invention the signal processor 62 is designed to sample eachAM signal at substantially regular intervals and substantiallysimultaneous times, process the signals to produce a display signal (notshown) which is then displayed on a readable display 63 such as:digital, hardcopy, video, software, computer memory displays or anaudible indicator.

While it is possible to actually measure the relative phases of theorthogonal components of the beam 12 after exiting the transducer, therelative phase shift can also be derived from the intensities of the AMsignals 35 and 36 shown in FIG. 8 without using complex and costlyapproaches as involved in direct phase measurements. Therefore, in thepresent invention, when the two AM signals 35 and 36 are separated froma single differential phase shifted signal using a properly orientedpolarizing beam splitter 80, the beam's polarization state is analyzedto obtain AM intensity signals. The AM signals 35 and 36 extracted fromthe beam's polarization state by the splitter 80 are transmitted to andused in the signal processor 62 where their complementary naturefacilitates rejection of common mode noise and minimizes effects oftemperature dependent intrinsic birefringence that may reside in thetransducing medium or other optical components within the system. Thisfeature of the present invention substantially enhances accuracy andpracticality of the system and represents an additional advancement overmuch of the prior art. The signal processor 62 performs these functionswhile converting the received AM signals 35 and 36 into a single, highlyaccurate voltage measurement. In addition to measuring the voltage of adevice, the invention may be used in conjunction with a device formeasuring current to provide information regarding power, power factorangle, and energy on the conductor of interest.

As mentioned, each AM signal, 35 and 36 (FIG. 8) is converted by aphoto-detector 43 and 44 (FIG. 1) into a electrical signal 45 and 46(FIG. 1) which can be processed by the signal processor 62. Thephotodetector comprises an optic-to-electronic conversion means forconverting said AM signals into analog electronic signals. Preferably,the analog electronic signals comprise low-level analog voltage signalsor current signals.

In the preferred embodiment of the present invention the electricalsignals 45 and 46 are electronic signals transmitted to the signalprocessor 62 which correspond to the intensity of the AM signals 35 and36. Thus, in the practice of the present invention, a series of AMsignals are manipulated by the signal processor 62, as each of theelectrical signals 45 and 46 corresponds to intensity of each AM signal35 and 36. The electrical signals 45 and 46 are sampled by the signalprocessor 62 at regular intervals and substantially simultaneously withone another. The sampled signals are the instantaneous intensity foreach AM signal, 35 and 36. These intensity signals will be discussedbelow as (A) and (B), respectively.

In the signal processor 62 (FIG. 1) the instantaneous intensity signalfor each beam component is sampled sequentially and stored, therebyforming a data base of stored signals which represents each AM signalover time. The stored signals are then converted into a displayablesignal regarding the voltage of E-field at the transducer 5 (FIG. 8).

In the preferred embodiment signals are manipulated in the followingmanner. First, an average intensity for each independent amplitudemodulated signal is calculated. This is done by summing theinstantaneous intensities of the signals which have been sampled over apre-determined time interval and dividing by the number of samples takenin the interval. In the preferred invention this is accomplished bytaking the average of the signals over time for each beam component bysumming the signals of each beam component and dividing the sum by thenumber of signal samples taken.

Mathematically, the average intensity for the AM signal (A) is expressedas follows: ##EQU1## where the average intensity is (), theinstantaneous AM signal is (A_(i)), the number of samples is (L), thesamples are taken and stored at uniform time intervals (i), and theaverage is calculated using samples between present time index n andpast time index (n-L). Similarly, the average intensity for the AMsignal (B) is expressed as follows: ##EQU2## where the average intensityis (), the instantaneous AM signal is (B_(i)), with the other valuesbeing as above.

Next, an adjusted instantaneous intensity for each signal is found bycomparing the most recent instantaneous signal intensity with theaverage signal intensity of the corresponding AM signal. Thus, for thebeam component corresponding to AM signal (A), the adjustedinstantaneous intensity (I_(n)) is:

    I.sub.n =A--A.sub.n

Where at (A_(n)) is the instantaneous intensity of AM signal (A) at thepresent time index. Similarly, for AM signal (B), the adjustedinstantaneous intensity (J_(n)) is:

    J.sub.n =B--B.sub.n

Where (B_(n)) is the instantaneous intensity of signal (B) at thepresent time. It will be recognized by those skilled in the art thatbecause signals (A) and (B) each represents a different axis on thepolarization ellipse, their amplitudes will change in oppositedirections from one another for a given change in polarization. Thuswhere the intensity of one signal increases there will be a decrease ofintensity of equal magnitude in the other signal. Therefore, theadjusted instantaneous intensity signals (I_(n)) and (J_(n)) must becomputed as indicated above in order to preserve sign.

The adjusted average instantaneous intensity signal for both signals (A)and (B) compensates for any temperature induced birefringence that mayexist within the transducer. Temperature induced birefringence causes achange in the intensity of the AM signals over time, as the transducerheat or cools. The variation in the intensity due totemperature-dependant intrinsic birefringence of the transducer appearsas a modulation or variation in the average intensity. Thus, bycomparing the instantaneous intensity with the average intensity of thesignals, and deducting the average intensity from the instantaneousintensity, temperature induced variations of the signal due to thebirefringence in the transducer are compensated for in the adjustedinstantaneous intensity signals (I_(n)) and (J_(n)).

An additional manipulation of the adjusted instantaneous intensitysignals (I_(n)) and (J_(n)) compensates for intensity fluctuations andother common mode noise. This is accomplished by comparing the averageof the adjusted instantaneous intensity signals (I_(n)) and (J_(n)) forthe signals (A) and (B). This comparison entails calculating the averagebetween (I_(n)) and the sign-reversed value of (J_(n)). ##EQU3## Thisaverage (K_(n)) is directly proportional to the voltage. This is sobecause the Pockels electro-optic effect induces a differential phaseshift in the orthogonal planes of the beam 12 (FIG. 8) which is directlyproportional to the E-field, and the E-field is directly proportional tovoltage. Thus, for a sampling of interest (n), the average instantaneousintensity signal (K_(n)) for the signals (A) and (B) is directlyproportional to the actual instantaneous voltage (V_(n)) betweenenergized conductor and ground, varying only by a scaling constant (K).##EQU4##

The scaling constant (K) is determined by applying a precisely knownvoltage to the device of interest and adjusting the scaling constant (K)until the value measured as the actual instantaneous voltage (V_(n)) isequivalent to the precisely known voltage being applied. In a typicalgeneral application of the present invention, shown in FIG. 1, thesensor head 4 is placed in an insulator 50 between a conductor 52 and agrounded conductor 48. When voltage is applied to the conductor 52 anE-field is created between the conductor 52 and the grounded conductor48, in the insulator 50. Determination of the scaling constant (K) isaccomplished by applying a precisely known voltage to the conductor 52.Once the scaling constant (K) is known the electro-optical voltagesensor system may be operated to determine additional actualinstantaneous voltages applied to conductor 52.

An alternative embodiment in accordance with the present invention isillustrated in FIG. 2, which is a diagram of the electro-optical voltagesensor system. Wherever practicable, components in the alternativeembodiment which are equivalent to those discussed above have the samereference numbers. In this embodiment there is shown a transmitter 1,which comprises a laser (not shown), a polarizer 6, and a wave plate 9.The laser emits an electromagnetic radiation beam 12. Theelectromagnetic radiation beam 12 passes first through a polarizer 6 andthen a wave plate 9, producing a polarized beam 12 which may becircular-polarized. The beam 12 enters into a PM optic fiber 18 througha collimator 20. The PM optic fiber 18 may further comprise alow-birefringence optic fiber. The optic fiber 18 routes the beam 12 toa sensor head generally designated at 16. It is important to note thatthe sensor head 16 in this alternative embodiment shown in FIG. 2differs from the sensor head 4 of the preferred embodiment shown in FIG.1.

The detector generally depicted at 8, as shown in FIG. 2, comprises apolarizing beam splitter 80, at least two photo-detectors 43, 44, and asignal processor 62. The beam 12 is routed from the sensor head 16through another PM optic fiber 19 through a collimator 23 and into thepolarizing beam splitter 80. Again, as shown in FIG. 3, the polarizingbeam splitter 80 separates the polarized beam 12 in accordance with therespective separate wave components propagating in the orthogonal axesof the polarization ellipse into AM signals 35, 36. In the embodimentshown in FIG. 2, the AM signals 35, 36 which are analog optic signals,are routed through multi-mode or single-mode optic fibers 37, 38 to thephotodetectors 43, 44.

The photo-detectors convert the AM signals 35, 36 into electricalsignals 45, 46 which can be analyzed in the signal processor 62.

Prior to discussing the sensor head 16, consider the collimators 20, 21,22, 23, 24, 25 in FIGS. 2, 7a, 7b, and 7c and collimators 26, 27 and 28in FIG. 8, which each are generally represented by the collimator 29,shown in FIG. 4. Generally, a collimator 29 comprises a lens 30 and atransparent end 31 which can pass a beam 12 into or out of the core 32of an optic fiber 40. A collimator used to couple light into the core ofan optic fiber is sometimes also referred to as a "coupler" in the art,but the term "collimator" is used herein for simplicity. In thepreferred embodiment, the optic fiber 40 is a PM optic fiber 18, whichmay also take the form of low-birefringence fiber.

Referring to FIGS. 7a, 7b, and 7c, these alternative embodiments of thesensor head 16 comprise a transducer 5 and a reflector 17. In FIG. 7athe beam 12 enters into the sensor head 16, through a collimator 21which is attached to a PM fiber 18. The beam 12 then passes through thetransducer 5 and upon being reflected by a beam reflector 17 the beamsis channeled along a path substantially parallel to the that which isfollowed through the transducer and enters into a second collimator 22.The second collimator 22 is connected to an optical fiber 19 by whichthe beam 12 is routed away from the sensor head 16.

Similarly, in the practice of the alternative embodiment of FIG. 7b, thebeam 12 enters into the sensor head 16, through a collimator 21 which isattached to a PM fiber 18, following which it is reflected by the beamreflector 17 into the transducer 5. The beam 12, after passing throughthe transducer 5 would enter into a second collimator 22 connected to anoptical fiber 19 and be routed away from the sensor head 16.

Likewise, in the practice of the alternative embodiment of FIG. 7c, thebeam 12 enters into the sensor head 16, following which it travelsthrough the transducer 5. The beam 12 is then reflected by the beamreflector 17 back through the transducer 5, and into a second collimator22 connected to an optical fiber 19. The beam 12 is then routed awayfrom the sensor head 16.

In the embodiments of FIGS. 1 and 2, the sensor heads 4 and 16 each havea cross-sectional area of only approximately fifty millimeters squared(50 mm²) or less, and a length of approximately twenty five centimeters(25 cm) or less, depending upon the particular apparatus in which thesensor head is embedded.

There are two alternative embodiments of the transducer 5 which varyaccording to intrinsic birefringence. The first embodiment of thetransducer 5, as shown in FIG. 5, is particularly appropriate where thetransducer material 5 does not exhibit intrinsic birefringence along thepropagation axis, shown as z. The beam 12 propagates along axis z. Thetransducer 5 in the presence of the E-field 90 causes differential phasemodulation in the orthogonal planes of the beam 12. The embodiment shownin FIG. 5 is preferred where the transducer 5 exhibits no intrinsicbirefringence. It is desirable to avoid intrinsic birefringence, as itis typically temperature dependant.

The second embodiment for the transducer 5 which exhibits intrinsicbirefringence is shown in FIG. 6. Here, the transducer (or transducingmedia) 5 and a second transducer (or second transducing media) 11, havematched intrinsic birefringence to one another along the opticalpropagation axes (or optical propagation orientation axes), shown as y'.The transducer 5 is aligned with the second transducer 11 such that theoptical propagation axes y' are rotated ninety degrees (180°) withrespect to one another. The E-field 90, aligned along the z axes,achieves a reverse polarity in the second transducer 11. The orientationof the beam 12 with respect to the optical propagation orientation axesy' is preserved by the rotation of the beam 12 by ninety degrees (90°).This rotation of the orientation is achieved by placement of a 90°polarization rotator 7 between the first transducer 5 and the secondtransducer 11. This configuration achieves cancellation of the effectsof intrinsic temperature dependant birefringence, while yielding adifferential phase shift in each transducer 5, 11. Thus, in thisembodiment, the beam 12 goes through the transducer 5 to the 90°polarization rotator 7, and thence through the second transducer 11.

As described above, the sensor head 4 is designed to measure voltage inthe conductor 52 when the sensor head is placed in an E-field created bythe conductor. In the preferred embodiment, the sensor head 4 is placedin an insulator 50 between the conductor 52 and a grounded conductor 48(see FIG. 1 and FIG. 2 and related discussions in the specification).Although an E-field originator 96 (shown generally in FIG. 1 and FIG. 2)is the preferred embodiment for generating an E-field in thisarrangement, other embodiments of the E-field originator are possible toaccomplish the same result.

FIG. 9 illustrates an alternative embodiment of the E-field originator96 of FIG. 1. In FIG. 9, a shielded joint 100 is the apparatus forgenerating an E-field in an environment in which the sensor head 4 ofthe present invention is placed. The shielded joint 100 is referred toas a "joint" because it is located at a position in which two conductors52 meet. The two conductors 52 each have an insulator 50 sheathed aboutthem which is linearly spaced apart from a conductor connector 104 thatsurrounds and connects the two conductors. Sheathed about the insulator50 is a semi-conducting jacket 109 which is, in turn, sheathed with acable ground shield 111. In addition, about the insulator 50 is placed alayer of shielded joint insulator 108 and a joint ground shield 102. Theshielded joint insulator 108 is positioned about the conductor connector104 and the conductors 52 but it is out of contact therewith. The jointground shield 102 is held in place and supported with end caps 115. Aconductive wire 106 and a ground shield connector 113 cause the jointground shield 102 to be common electrically with the cable ground shield111. Sensor head 4 is placed in the shielded joint insulator 108 betweenthe joint ground shield 102 and the conductor connector 104. Opticalfibers 18, 37, and 38 route an electromagnetic beam to and from thesensor head 4. The E-field of interest, and into which the sensor head 4is introduced, arises between the joint grounding shield 102 andconductor connector 104 when an electric voltage is present upon theconductors 52.

Another expression of the E-field originator 96 is the terminator 118shown in FIG. 10. In the preferred embodiment showing a terminator 118,a conductor 52 is enclosed within insulation 50 forming a transitionspan. The insulation 50 is enclosed within a voltage gradient controlmeans 120. A cable ground shield 111 may partially enclose the voltagegradient control means 120. The sensor head 4 is disposed between theconductor 52 and the voltage gradient control means 120. Optical fibers18, 37, 38 route the beam to and from the sensor head 4. The E-field ofinterest, and into which the sensor head 4 is introduced, arises betweenconductor 52 and the voltage gradient control means 120 when a voltageis present upon the conductor 52.

Another embodiment of the E-field originator 96 of FIGS. 1 and 2 is thethrough-hole insulator 124, here shown as a transformer bushing in FIG.11. In the preferred embodiment showing a through-hole insulator 124, aconductor 52 is enclosed within insulation 50, which is at leastpartially enclosed within a grounded conductor 48. The through-holeinsulator shown here as a transformer bushing has a mounting flange 126for mounting the bushing and creepage distance skirts 128 which extendfrom the grounded conductor 48. The sensor head 4 is disposed betweenthe conductor 52 and the grounded conductor 48. Optical fibers 18, 37,38 route the beam to and from the sensor head 4. The E-field ofinterest, and into which the sensor head 4 is introduced, arises betweenthe conductor 52 and the grounded conductor 48 when a voltage is presentupon the conductor 52.

Another embodiment of the E-field originator 96 of FIGS. 1 and 2 is theshielded cable or bus 129, shown in FIG. 12. In the preferred embodimentof a shielded cable or bus 129, a conductor 52 is enclosed withininsulation 50, which is enclosed within a grounded conductor 48. Thesensor head 4 is disposed between the conductor 52 and the groundedconductor 48. Optical fibers 18, 37, 38 route the beam to and from thesensor head 4. The E-field of interest, and into which the sensor head 4is introduced, arises between the conductor 52 and the groundedconductor 48 when a voltage is present upon the conductor 52. WhereasFIG. 12 shows a bus having a round cross-section, an alternative andequivalent embodiment comprises a rectangular (or square) cross-section.

Another embodiment of the E-field originator 96 shown in FIGS. 1 and 2is the gas or oil-insulated switchgear 132, shown in FIG. 13. In thepreferred embodiment showing a gas or oil-insulated switchgear 132, aconductor 52 is enclosed within gas or oil insulation 134, by a groundedconductive or semi-conductive containment means 140 for containing thegas or oil insulation. The sensor head 4 is disposed between theconductor 52 and the grounded containment means 140. Optical fibers 18,37, 38 route the beam to and from the sensor head 4. The E-field ofinterest, and into which the sensor head 4 is introduced, arises betweenthe conductor 52 and the grounded containment means 140 when a voltageis present upon the conductor 52.

Another embodiment of the E-field originator 96 is the duct-enclosed bus142, shown in FIG. 14. In the preferred embodiment of a duct-enclosedbus 142, a conductor 52 is enclosed within insulation 50 of sufficientthickness to minimize possibility of flash-over. The insulated bus isenclosed within a grounded duct or other grounded at leastsemi-conducting means 144. Those skilled in the art will appreciate thatby the term "at least semi-conducting", it is meant all semi-conductingand conducting means, by which the bus can be grounded. The sensor head4 is disposed between the conductor 52 and the grounded semi-conductingmeans 144. Optical fibers 18, 37, 38 route the beam to and from thesensor head 4. The E-field of interest, and into which the sensor head 4is introduced, arises between the conductor 52 and the groundedsemi-conducting means 144 when a voltage is present upon the conductor52.

Another embodiment of the E-field originator 96 is the clamp-onapparatus 147, shown in FIG. 15. In the preferred embodiment of theclamp-on apparatus, a conductor 52 is enclosed within a coaxialconductive grounded surrounding means 149. In the practice of theinvention, the conductor 52 is preferably a nonshielded transmissionline, a nonshielded cable, or a nonshielded bus. The conductor 52 isseparated from the conductive grounded surrounding means 149 byinsulated standoff means 151. The sensor head 4 is disposed between theconductor 52 and the coaxial conductive grounded surrounding means 149.Optical fibers 18, 37, 38 route the beam to and from the sensor head 4.The E-field of interest, and into which the sensor head 4 is introduced,arises between the conductor 52 and the coaxial conductive groundedsurrounding means 149 when a voltage is present upon the conductor 52.

FIG. 16 is an alternative embodiment of the electro-optical voltagesensor of FIG. 1, having a current detector 153 disposed substantiallyin common with the sensor head. The current detector 153 detects thecurrent, not shown, passing through the conductor 52 by detecting themagnetic field, not shown, associated with the current. The currentdetector provides a current signal, not shown, which is routed through acurrent signal routing member 156 to the signal processor 62. In oneembodiment the optical current signal routing member 156 consists ofoptic fiber, photodetection means, and electrical connecting means, noneof which are shown. Where voltage (V) and current (I) are known, thepower (P) can be determined by the signal processor by multiplying thecurrent (I) by the voltage (V) as follows:

    P=IV

A combination of at least two sensors of the present invention may beused to achieve a line-to-ground voltage measurement on separateconductors of a multi-source system. "Multi-source system" is meant a toinclude both a multi-phase as well as a multi-line system. In such asystem, line-to-ground measurements may also be used to calculateline-to-line voltages through simple subtraction of line-to-groundvalues.

The present invention represents a significant advance over the priorapparatus, methods and art of voltage measurement. The present inventiondoes not require any additional electronics to bias the transducingmaterial to determine voltage, such as a voltage divider. Voltage isdetermined in the present invention by utilizing the E-field that existswithin many types of devices to transmit and distribute power, oftenthese power distribution devices are co-axial, which simplifies theapplication of the present invention. The E-field, which is proportionalto voltage, is used to bias a transducing element in order to induce adifferential phase shift in the orthogonal planes of the beam, which asmodulated optic signals, is proportional to voltage.

It is noted that many of the advantages of the present invention accruedue to the simplified structure of the sensor head, which issufficiently small so as to be conveniently installed in devices inwhich E-fields arise, or built in as part of a sensor.

Although the prior art apparatus and methods for voltage measurementhave attempted to use the electro-optical effect in materials havingeither a Pockels or Kerr coefficient, they have typically required aseparate compensator crystal with a known reference voltage or aseparate voltage divider directly connected to the energized conductorin order to make a voltage measurements. The result has been deviceswhich were bulky and required additional electronics for measuring theknown reference, or required extra hardware presenting size, weight,expense, reliability, and other problems.

By using the sensor head of the present invention, which may beinstalled or built into the described voltage transmission anddistribution apparatus, voltage measurement is achieved without the useof a large, dedicated, stand-alone voltage division device. Real estatewithin Power substations is at a premium, thus this sensor system offersa substantial economic advantage due to space savings. In addition,contact with the energized conductor is substantially reduced and inmost cases altogether eliminated with the practice of the presentinvention. This is advantageous, as an energized conductor can presentsignificant life and health risks among other hazards where highvoltages are involved. Further, the practice of the present inventiondoes not interfere with the apparatus being measured, and avoids variousother problems associated with the use of the voltage dividers in theprior art.

Those skilled in the art will appreciate from the preceding disclosurethat the objectives stated above are advantageously achieved by thepresent invention.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the invention.Numerous modifications and alternative arrangements may be devised bythose skilled in the art without departing from the spirit and scope ofthe present invention and the appended claims are intended to cover suchmodifications and arrangements.

We claim:
 1. A sensor for sensing voltage in an E-field developedbetween a conductor means and a grounded conductor means, said voltagesensor comprising:transmitting means for transmitting at least one beamof polarized electromagnetic radiation, said beam having at least twocomponents propagating along at least two orthogonal planes,respectively, to form at least two orthogonal beam components; sensingmeans configured for disposition between the conductor means and thegrounded conductor means but out of contact with the conductor means forsensing the presence of the E-field and inducing a differential phaseshift on the orthogonal beam components when said sensing means is insaid E-field as the beam travels along a path through the sensing means;reflector means disposed adjacent to the sensing means and positionedfor receiving the at least one beam from the sensing means when the beamhas passed out of the sensing means and for channeling the been along apath substantially parallel to the path through the sensing means;detecting means for detecting said differential phase shift of saidorthogonal beam components; and first and second routing means, saidfirst routing means for routing said beam from said transmitting meansto said sensing means, and said second routing means for routing saidbeam components from said sensing means to said detecting means.
 2. Thesensor as in claim 1, wherein said first and second routing means eachcomprises:at least one translucent element for routing said beam andsaid beam components.
 3. The sensor as in claim 2, wherein each of saidat least one translucent element comprises at least one optic fiber. 4.The sensor as in claim 3, wherein said at least one optic fibercomprises an optic fiber selected from the group consisting of:(1) asingle-mode optic fiber, and (2) a multi-mode optic fiber.
 5. The sensoras in claim 4, wherein at least one of said optic fibers comprises apolarization maintaining (PM) optic fiber for routing said beam withdecreased loss of polarization of the beam.
 6. The sensor as in claim 4,wherein at least one of said optic fibers comprises a low birefringenceoptic fiber for routing said beam with decreased loss of polarization ofthe beam.
 7. The sensor as in claim 1, wherein said sensing meansfurther comprises:beam splitting means for separating orthogonal beamcomponents oriented along the axes of the beam's polarization ellipse(whose ellipticity varies between -1 and +1 in proportion to voltage)and forming at least two independent amplitude-modulated signalstherefrom, respectively.
 8. The sensor as in claim 1, wherein saiddetecting means further comprises:beam separation means for separatingorthogonal beam components oriented along the axes of the beam'spolarization ellipse (whose ellipticity varies between -1 and +1 inproportion to voltage) and forming at least two independentamplitude-modulated signals therefrom, respectively; and processingmeans for converting said independent-amplitude modulated signals intoat least one display signal proportional to the voltage of said E-field.9. The sensor as in claim 8, wherein said processing means furthercomprises:optic-to-electronic conversion means for converting said AMsignals into analog electronic signals, wherein said analog electronicsignals comprise signals selected from the group consisting of: (1)low-level analog voltage signals, and (2) current signals.
 10. Thesensor as in claim 9, wherein said processing means furthercomprises:analog-to-digital conversion means for converting said analogelectronic signals into at least one digital electronic signal.
 11. Thesensor of claim 1, wherein said sensor further detects the presence andmagnitude of a magnetic field caused by a current, wherein said sensorfurther comprises:current detection means for detecting said magneticfield and determining the current therefrom, said current detectionmeans being disposed substantially in common with said sensing means.12. A method of using a sensor head as part of a system for sensingvoltage in an E-field developed between a conductor means and a groundedconductor means, said sensor head having an electro-optical sensingmeans for inducing a differential phase shift of at least two beamcomponents propagating in at least two orthogonal planes aselectromagnetic wave components of a beam of electromagnetic radiationfrom transmitting means when said sensor head is in an E-field, saidmethod comprising the steps of:(a) disposing the sensor head in theE-field between the conductor means and the grounded conductor means andout of contact with the conductor means; (b) developing anelectromagnetic radiation beam having at least two beam componentspropagating along at least two orthogonal planes, respectively; (c)routing said beam from said transmitting means along a path through saidsensor head to induce a differential phase shift of the beam components;(d) routing said beam from said sensor head to processing meansincluding reflecting the beam to travel along a path generally parallelto the path through the sensor after the beam has passed out of thesensor head; and (e) processing said beam components by converting thecomponents into at least one electrical signal whose value isproportional to the voltage of the E-field at the sensor head.
 13. Themethod as defined in claim 12, wherein the routing of step (c) furthercomprises:separating specific beam components from said beam into atleast two independent amplitude modulated signals, each independentamplitude modulated signal comprising a beam component.
 14. The methodas defined in claim 12, wherein the processing of step (d) furthercomprises the steps of:(e) sampling the electrical signal atsubstantially regular intervals, said sampling occurring substantiallysimultaneously for so all beam components; (f) storing said sampledsignal; and (g) manipulating said sampled signal of each beam componentso as to convert said signals into a display signal, said display signalbeing proportional to the voltage of said E-field.
 15. The method ofclaim 14, in which the manipulating of step (g) further comprises thesteps of:(h) determining an average electrical signal for each beamcomponent by summing the sampled electrical signals of each beamcomponent over a specified time interval and dividing said sum by thenumber of samples of the beam component taken in that interval; (i)determining an adjusted instantaneous signal for each beam component bycomparing the most recent electrical signal of the beam componentsampled with the most recently calculated average electrical signal ofthe beam component and taking the difference thereof; (j) determining anadjusted average instantaneous signal for all beam components by summingthe adjusted instantaneous signal of each beam component and dividingthe same by the number of beam components; and (k) determining a scaledadjusted average instantaneous signal by proportionally increasing thescaled adjusted average instantaneous signal by multiplying saidadjusted average instantaneous signal by a scaling constant; said scaledadjusted average instantaneous signal is a display signal representingvoltage between ground and the energized conductor on which the voltagemeasurement is being taken.
 16. The method of claim 14, furthercomprising the step of:(1) displaying said display signal on displaymeans, wherein said display means for displaying said display signal isselected from the group consisting of:(1) a digital display; (2) ahardcopy display; (3) a video display; (4) a software display; (5) acomputer memory; and (6) an audible indicator.
 17. A sensor for sensingvoltage in an E-field developed between a conductor means and a groundedconductor means with insulator means disposed therebetween, said voltagesensor comprising:transmitting means for transmitting at least one beamof polarized electromagnetic radiation, said beam having at least twocomponents propagating along at least two orthogonal planes,respectively, to form at least two orthogonal beam components; sensingmeans configured for disposition in the insulator means between theconductor means and the grounded conductor means, but out of contactwith the conductor means, for sensing the presence of the E-field andinducing a differential phase shift on the orthogonal beam componentswhen said sensing means is in said E-field as the beam travels along apath through the sensing means; detecting means for detecting saiddifferential phase shift of said orthogonal beam components; and firstand second routing means, said first routing means for routing said beamfrom said transmitting means to said sensing means, and said secondrouting means for routing said beam components from said sensing meansto said detecting means.
 18. A sensor for sensing voltage in an E-fielddeveloped by a conductor and a grounded conductor,comprising:transmitting means for transmitting at least one beam ofpolarized electromagnetic radiation, said beam having at least twocomponents propagating along at least two orthogonal planes,respectively, to form at least two orthogonal beam components; sensingmeans disposed between the conductor and the grounded conductor but outof contact with the conductor for sensing the presence of the E-fieldand inducing a differential phase shift on the orthogonal beamcomponents when said sensing means is in said E-field as the beamtravels along a path through the sensing means; beam splitting means forseparating orthogonal beam components oriented along the axes of thebeam's polarization ellipse and forming at least two independentamplitude modulated signals therefrom, respectively; first and secondrouting means, said first routing means for routing said beam from saidtransmitting means to said sensing means, and said second routing meansfor routing said beam components from said sensing means to saiddetecting means; and detecting means for detecting said differentialphase shift of said orthogonal beam components based on the at least twoindependent amplitude modulated signals.